Mute notes/ Dead Notes in slap bass

The standard way to pull off a dead note/ mute note is to hold down the string on the fretboard with your left hand without actually pressing down to play the string, yes? Ok, my question is when it comes to slap bass(slap and pop) does the same method HAVE to be applied? According to a friend of mine some people like to mute the string with the back of their palm instead. Which technique is proper? or are both acceptable?

Flea uses the palm style. I'd recommend learning both. Even with parallel thumb I can mute that way. There's some Flea lines you just can't play (or at least comfortably) without it. It definitely frees up the left hand for some acrobatics.

Yeah, I agree, there is no right or wrong way. Both may have slighty different sounds and may work better in different situations, you are the master, you choose what you want to do & when etc.
Even watch Robert Trujillo when he slaps (more Infectious Grooves than Metallica obviously) he is a very different technique, but it sounds awesome.

If you ever watch a guy that hasn't had any lessons but either a natural or practiced hard on their own, you will see all sorts of different techniques, none of them wrong.